Using Drosophila melanogaster, studies are being made on the relationship of whole-body mutants to fractionals of mosaics after treatment with radiation and mutagenic chemicals. Specifically, the experiments are directed towards an understanding of the interactions of broken chromosomes, and their eventual loss during early development with chemically induced changes. The methods involve the treatment of young males with diethyl-nitrosamine for two days. This compound is known to induce a very high frequency of fractional mutations but a zero (or very low) frequency of chromosome breaks. This treatment is followed by a dose of radiation of 2,000 R and 4,000 R to induce chromosome breaks. Several different kinds of genetic effects have been measured including the frequency of induced sex-linked mosaic lethals, the frequency of mosaics at a specific locus, dumpy, and the frequency of mosaics for a set of selected loci on the X-chromosome and the autosomes. The data collected so far indicate that there is a slight decline in the recovery of these events in the combined treatments, compared to the frequency of their occurrence after the chemical treatment alone. The significance, both biological and statistical, of these observations is being pursued.